Q: iphoto forgets its library

running the latest yosemite on a 2010 imac.  we have two users with separate log-ins to keep one of us from accidentally corrupting the other's data.  a guru placed our iphoto library so that it could be shared and either of us could edit/corrupt at will.

one user has had no problems, ever.  the other has, on more than one occasion.  for that user, when iphoto is launched, it does not 'see' the correct library (13,000 photos) and load it automatically.  instead it asks the user to 'choose a library. ' it offers only one, and not the real one:  'iphoto library (default)' which contains maybe a few hundred pics.  once, it (the default library) contained only one picture.

i know just barely enough to get in trouble, but in the past i have messed around and somehow gotten the correct library back in service.  it works for a while (months, maybe) and goes out again.  since i don't know what i'm doing, i neither know what exactly did the fix nor do i remember how to repeat it when the problem recurs.

if anybody can provide a remedy that will be permanent, great.  one i can save and reuse will also be appreciated.